This invention relates to nuclear fuel material and has particular relationship to such material containing plutonium. The expression nuclear fuel material as used in this application means the nuclear fuel substance itself, such as the substance of which fuel pellets or the like are formed, as distinct from the fuel elements or fuel assemblies, for example, as shown in Sease U.S. Pat. No. 3,778,348 or Stoll U.S. Pat. No. 3,778,885. The expression "nuclear fuel material" means the nuclear fuel particles coating the inner wall of Stoll's fuel elements not the elements themselves.
Plutonium is produced as a by-product to power production in conventional reactors fueled with uranium or with uranium and plutonium. While this invention is applicable to most types of converter reactors (for example, Canadian heavy-water and steam-cooled heavy water reactors, and some species of gas-cooled), this application, in the interest of concreteness, deals with the most common type, the light water reactor (LWR). Fuel discharged from such reactors generally contains about 0.9% plutonium of which about 0.6% is fissile plutonium. This material is reprocessed and can be reused after augmentation with additional fissile material. During reprocessing fission products are removed and usually there is chemical separation of the residual plutonium from the residual uranium. The plutonium is usually recovered in a plutonium nitrate solution, or with some subsequent processing, plutonium oxide powder. Such by-product plutonium either prior to or subsequent to conversion to the oxide or other form is blended with a suitable quantity of uranium, and this mixed fuel, usually mixed oxide, material is subsequently fabricated into suitable fuel elements. Such fuel elements can be reintroduced into the reactor wherein the plutonium was generated during refueling or into other reactors to serve as fuel material, providing the mechanism for fissioning and thereby producing heat. Fast breeder reactors use plutonium as the primary fuel material and during the course of power generation produce more plutonium than is consumed. This excess plutonium, subsequent to reprocessing, can similarly be used as the fuel in other reactors.
Since its discovery, plutonium has been known to be one of the most toxic materials known to man. Stringent control levels have been dictated by government regulatory agencies to minimize risk to processing and fabrication plant personnel as well as to the general public. A maximum body burden for soluble compounds has been set at 0.04 microcuries (about 0.6 micrograms of Pu-239), and the maximum permissible concentration in the breathing atmosphere for 40-hour per week exposure has been set at 2.times.10.sup.-12 microcuries per cubic centimeter of air if soluble plutonium compounds are involved, and 4.times.10.sup.-11 microcuries per cubic centimeter, for insoluble compounds. This latter criteria translates into a maximum permissible lung burden of 0.016 microcuries (or 0.26 micrograms of Pu-239). These restrictions have led to a distinctive plutonium processing and fabrication technology characterized by containment structures, semi-remote handling, extensive ventilation precautions to minimize the probability for plutonium to escape from the enclosures to the breathing air, and extensive monitoring to detect escaped plutonium as soon as possible. The associated processing and fabrication of fuel pellets, fuel elements (usually rods), and fuel assemblies for plutonium is considerably more complex and costly than for the sister element uranium.
This has led to the practice, in accordance with the teachings of the prior art, to concentrate as much plutonium as practicable into plutonium reactor fuel rods so that the number of fuel rods required to be fabricated with the use of the costly plutonium technology is minimized. For example, in light water reactors, a fuel rod that is fabricated with plutonium, under prior art practice, consists of natural uranium (0.72% fissile U-235), depleted uranium (0.2 to 0.4% U-235), or recycled uranium from the reprocessing operation (U-235 concentration up to about 1%), with plutonium being added in sufficient quantity to make a useful reactor fuel (total fissile material content between 2.2 and 5%). This is commonly called a U-235 replacement method of plutonium recycle. Within the concentration range dictated by prior art, fissile plutonium (in association with the non-fissile plutonium isotopes normally generated in reactor-produced plutonium) has less reactivity value than U-235, hence for each atom of U-235 displaced, approximately 1.25 fissile atoms of plutonium must be added (together with the associated non-fissile isotopes). More explicitly, if a fuel rod containing only 3.3% enriched uranium is required in a certan reactor core location, this is replaced in accordance with the teachings of the prior art with a mixed-oxide fuel rod that contains natural uranium (0.72% U-235) plus approximately (3.30-0.72).times.1.25=3.225% fissile plutonium. However, such plutonium addition and U-235 replacement causes local power peaking because with neutron fission cross section of plutonium is higher than that of U-235. Local introduction of plutonium therefore requires modification to surrounding fuel elements or to location of control materials so as to keep power peaking to acceptable levels. Local introduction of plutonium, therefore, requires significant alteration of core design and evaluation of fuel behavior.
Plutonium fuel material in accordance with the teachings of the prior art are, because of the costly processing, handling, and maintenance which they demand, costly. In addition, in reactors containing rods having the relatively high content of plutonium of the prior art there exists a non-uniform spacial distribution of energy generation and, in addition, fuel cycle equilibrium is not attainable in such reactors.
It is an object of this invention to overcome the above-described disadvantages of the prior art and to provide nuclear fuel material including plutonium whose processing, handling, and maintenance cost shall be substantially lower than that of prior-art plutonium-containing fuel material. It is a further object of this invention to provide a method of making such plutonium-containing nuclear fuel material. It is also an object of this invention to provide a nuclear reactor including plutonium-containing nuclear fuel material which shall not manifest the local power peaking of prior-art reactors containing plutonium nuclear fuel and in whose use an equilibrium fuel cycle shall be attainable.